1. Field of the Invention
The present invention relates to a cutting tool for milling having an insert of a cubic boron nitride sintered body which is employed for milling cast iron components or the like, and more particularly, it relates to a cutting tool for milling such as a milling cutter or an end mill which enables high-speed milling of cast iron components with long life.
2. Description of the Prior Art
In general, cast iron components for automobile engines or electrical appliances are face-milled or end-milled with a high-speed steel tool, a cemented carbide tool, a coated tool, a ceramics tool or a tool of a cubic boron nitride (hereinafter referred to as CBN) sintered body.
FIGS. 1A and 1B show a face milling tool, to which the present invention is applied. In this face milling tool, a plurality of throwaway tips prepared by brazing CBN sintered bodies 2 to only upper surfaces in the vicinity of single ends of bases 1 consisting of cemented carbide blocks are radially mounted on the outer periphery of a cutter body 5 as shown in FIG. 1B, with pressers 6 and clamp screws 7. In relation to such a face milling tool, Japanese Patent Laying-Open No. 8-141822 (1996) in the name of the assignee proposes a throwaway tip consisting of a CBN sintered body and a cutter for milling, which enables high-speed face milling of components consisting of gray cast iron with long life by optimizing the shapes of an insert, a minor insert, a negative land etc. of the throwaway tip.
Cutting speeds V practically employed for face-milling cast iron with a cemented carbide or coated tool and a ceramics tool are about 150 to 250 m/min. and about 400 m/min. respectively. On the other hand, a CBN tool which is excellent in wear resistance and applicable to high-speed cutting, such as the throwaway tip for milling proposed in the aforementioned gazette, for example, is capable of working cast iron at a cutting speed V of 500 to 1500 m/min. under a dry condition.
A cutting speed V practically employed for end-milling cast iron with a high-speed steel, cemented carbide or coated tool is about 30 to 100 m/min. On the other hand, a CBN tool is capable of working cast iron at a cutting speed V of 100 to 1500 m/min. under a dry condition.
As hereinabove described, the CBN sintered body tool is capable of face-milling cast iron at the cutting speed V of 500 to 1500 m/min. under a dry condition. Under a wet condition, however, the CBN sintered body tool is employed at a practical cutting speed V within the range of 500 to 700 m/min. If the cutting speed V exceeds this range, the insert of the CBN sintered body tool is heat-cracked to remarkably reduce the tool life.
This is because the insert which is heated to an extremely high temperature when coming into contact with workpieces is rapidly quenched in slipping and heat-cracked by the heat cycle applied thereto in case of cutting cast iron at a high speed under a wet condition, while the CBN sintered body, which has higher heat conductivity and a lower thermal expansion coefficient as compared with cemented carbide or ceramics, can withstand thermal shock with small temperature difference in a heat cycle under a dry condition.
As hereinabove described, the CBN sintered body tool is capable of end-milling cast iron at the cutting speed V of 100 to 1500 m/min. under a dry condition. Under a wet condition, however, the CBN sintered body tool is employed at a practical cutting speed V within the range of 100 to 300 m/min. If the cutting speed V exceeds this range, the insert is heat-cracked to remarkably reduce the tool life similarly to the case of face milling.
It is conceived that the tool life is reduced under a wet condition since the heat conductivity of the conventional CBN sintered body is less than 400 W/m.K under 20.degree. C. and a remarkable temperature gradient is caused in the vicinity of its cutting edge and high tensile stress is applied to the insert in quenching with respect to the temperature difference in the heat cycle under the wet condition due to the low heat conductivity even if the thermal expansion coefficient exceeds 4.0.times.10.sup.-6 /K in the temperature range of 20.degree. C. to 600.degree. C., and heat cracking readily results from repetition of remarkable expansion and contraction due to the high thermal expansion coefficient.
If the cutting speed V of the CBN sintered body tool is increased under a wet condition similarly to that under a dry condition, the tool life is unpreferably reduced to increase the working cost in both of face milling and end milling. Therefore, the CBN sintered body tool must cut workpieces which are remarkably deformed or distorted by heat generated in cutting or components to be protected against even the slightest deformation caused by heat generated in cutting under a dry condition or at a low cutting speed V causing no heat cracking under a wet condition.
However, various types of machining equipment rotatable at a high speed are recently developed one after another, and high-speed cutting is necessary and inevitable for improving working efficiency. In relation to such machining equipment, therefore, awaited is provision of an insert tool capable of coping with dry working for suppressing influence exerted on workpieces by increase of the cutting temperature.
While automobile engine components or the like may be milled under a dry condition, the working is performed under a condition identical to the wet condition is a pre-step includes wet working such as rough working or perforation due to a cutting fluid remaining on the working spots, and hence the insert of the tool is still heat-cracked and hence sufficient life cannot be obtained in this case.